Unify common message handler traits into one trait

[TheBlueMatt]

Instead of having each *MessageHandler implement the same
peer_connected, peer_disconnected, provided_init_features,
and provided_node_features methods, here we pull them all out
into one common trait and make all the message handlers extend it.

Since get_and_clear_pending_msg_events was also common we also
move that into the same trait, dropping the
MessageSendEventsProvider trait.

As requested by @tnull.

[tnull]

Cool! Unfortunately currently breaks RUSTFLAGS=--cfg=async_payments

[TheBlueMatt]

Oops, fixed.

[tnull]

Oops, fixed.

Unfortunately benchmark still failing:

error[E0432]: unresolved import `crate::events::MessageSendEvent`
     --> /home/runner/work/rust-lightning/rust-lightning/lightning/src/ln/channelmanager.rs:15865:29
      |
15865 |     use crate::events::{Event, MessageSendEvent};
      |                                ^^^^^^^^^^^^^^^^ no `MessageSendEvent` in `events`
      |
      = help: consider importing this enum through its public re-export instead:
              crate::ln::channelmanager::MessageSendEvent

warning: unused import: `Mutex`
     --> /home/runner/work/rust-lightning/rust-lightning/lightning/src/ln/channelmanager.rs:15881:25
      |
15881 |     use crate::sync::{Arc, Mutex, RwLock};
      |                            ^^^^^
      |
      = note: `#[warn(unused_imports)]` on by default

warning: unused import: `RouteHop`
    --> /home/runner/work/rust-lightning/rust-lightning/lightning/src/routing/scoring.rs:3678:44
     |
3678 |     use crate::routing::router::{bench_utils, RouteHop};
     |                                               ^^^^^^^^

warning: unused imports: `ChannelFeatures` and `NodeFeatures`
    --> /home/runner/work/rust-lightning/rust-lightning/lightning/src/routing/scoring.rs:3680:31
     |
3680 |     use crate::types::features::{ChannelFeatures, NodeFeatures};
     |                                  ^^^^^^^^^^^^^^^  ^^^^^^^^^^^^

warning: unused variable: `network_graph`
    --> /home/runner/work/rust-lightning/rust-lightning/lightning/src/routing/scoring.rs:3684:8
     |
3684 |         let (network_graph, mut scorer) = bench_utils::read_graph_scorer(&logger).unwrap();
     |              ^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_network_graph`
     |
     = note: `#[warn(unused_variables)]` on by default

[TheBlueMatt]

Grr, I really need to fix the CI script so I can run it locally top-to-bottom :/

[TheBlueMatt]

Gonna at least wait for #3575 and maybe one of the followups for it. Should be pretty straightforward to rebase this.

[wpaulino]

Needs a rebase now that the dependent PR was merged

[TheBlueMatt]

Rebased, I think we can land this now.